Electrical signals cause a heart to beat. In a healthy patient, regular heart beats pump blood through the cardiovascular system. The human cardiovascular system is responsible for receiving oxygen-deprived blood into the heart from the venous system of the body, delivering the oxygen-deprived blood to the lungs to be replenished with oxygen, receiving the oxygenated blood from the lungs back into the heart, and delivering the oxygenated blood to the body via the arterial vasculature. This process is regulated within the heart by electrical pulses that control operation of the heart's receiving and pumping chambers.
In a healthy heart, the sinoatrial node of the heart generates electrical pulses in a consistent and regulated fashion to regulate receiving and pumping blood in the heart's chambers. The electrical impulses propagate as activation wavefronts across the atria, the upper chambers of the heart, and cause cells of the atria to depolarize and contract, which forces blood from the atria to the ventricles, the lower chambers of the heart. The ventricles receive the blood from the atria, and the wavefront, after passing through the atrioventricular node and moving to the Purkinje system, moves to cells of the ventricles causing the ventricles to contract and pump the blood to the lungs and to the rest of the body.
Various aspects of cardiac activity (e.g., heart rate, arrhythmias) can be detected by measuring, recording, and analyzing cardiac electrical signals, such as an electrocardiogram (ECG) signal. One way of measuring ECG signals involves attaching electrodes, typically ten, externally to a patient's skin and sensing the electrical signals that form the ECG waveform.
Implantable monitoring systems can be implanted under the skin with electrodes that sense subcutaneous electrical signals, including ECG signals, which are analyzed as being indicative of cardiac activity. In such systems, the electrodes also receive extraneous non-cardiac electrical signal information, which is typically filtered out to produce a more readable ECG. Non-cardiac electrical signals can be generated by muscle tissues during physical activity, for example. In some examples, an implantable loop recorder (ILR) can record and quantify patient heart electrical activity.